Phosphorylation of proteins is a fundamental mechanism for regulating diverse cellular processes. While the majority of protein phosphorylation occurs at serine and threonine residues, phosphorylation at tyrosine residues is attracting a great deal of interest since the discovery that many oncogene products and growth factor receptors possess intrinsic protein tyrosine kinase activity. The importance of protein tyrosine phosphorylation in growth factor signal transduction, cell cycle progression and neoplastic transformation is now well established (Hunter et al., Ann. Rev. Biochem. 54:987-930 (1985), Ullrich et al., Cell 61:203-212 (1990), Nurse, Nature 344:503-508 (1990), Cantley et al., Cell 64:281-302 (1991)).
Biochemical studies have shown that phosphorylation on tyrosine residues of a variety of cellular proteins is a dynamic process involving competing phosphorylation and dephosphorylation reactions. The regulation of protein tyrosine phosphorylation is mediated by the reciprocal actions of protein tyrosine kinases (PTKases or PTKS) and protein tyrosine phosphatases (PTPs). The tyrosine phosphorylation reactions are catalyzed by PTKs. Tyrosine phosphorylated proteins can be specifically dephosphorylated through the action of PTPs. The level of protein tyrosine phosphorylation of intracellular substances is determined by the balance of PTK and PTP activities. (Hunter, T., Cell 58:1013-1016 (1989)).
2.1. PTKs
The protein tyrosine kinases (PTKS; ATP:protein-tyrosine O-phosphotransferase, EC 2.7.1.112) are a large family of proteins that includes many growth factor receptors and potential oncogenes. (Hanks et al., Science 241:42-52 (1988)). Many PTKs have been linked to initial signals required for induction of the cell cycle (Weaver et al., Mol. and Cell. Biol. 11(9):4415-4422 (1991)). PTKs comprise a discrete family of enzymes having common ancestry with, but major differences from, serine/threonine-specific protein kinases (Hanks et al., supra). The mechanisms leading to changes in activity of PTKs are best understood in the case of receptor-type PTKs having a transmembrane topology (Ullrich et al. (1990) supra). The binding of specific ligands to the extracellular domain of members of receptor-type PTKs is thought to induce their oligomerization leading to an increase in tyrosine kinase activity and activation of the signal transduction pathways (Ullrich et al., (1990) supra). Deregulation of kinase activity through mutation or overexpression is a well-established mechanism for cell transformation (Hunter et al., (1985) supra; Ullrich et al., (1990) supra).
2.2. PTPs
The protein phosphatases are composed of at least two separate and distinct families (Hunter, T.(1989) supra) the protein serine/threonine phosphatases and the protein tyrosine phosphatases (PTPs; protein-tyrosine-phosphate phosphohydrolase, EC 3.13.48)). The PTPs are a family of proteins that have been classified into two subgroups. The first subgroup is made up of the low molecular weight, intracellular enzymes that contain a single conserved catalytic phosphatase domain. All known intracellular type PTPs contain a single conserved catalytic phosphatase domain. Examples of the first group of PTPs include (1) placental PTP 1B (Charbonneau et al., Proc. Natl. Acad. Sci. USA 86:5252-5256 (1989); Chernoff et al., Proc. Natl. Acad. Sci. USA 87:2735-2789 (1990)), (2) T-cell PTP (Cool et al. Proc. Natl. Acad. Sci. USA 86:5257-5261 (1989)), (3) rat brain PTP (Guan et al., Proc. Natl. Acad. Sci. USA 87:1501-1502 (1990)), (4) neuronal phosphatase (STEP) (Lombroso et al.,Proc. Natl. Acad. Sci. USA 88:7242-7246 (1991)), and (5) cytoplasmic phosphatases that contain a region of homology to cytoskeletal proteins (Guet al., Proc. Natl. Acad. Sci. USA 88:5867-57871 (1991); Yang et al., Proc. Natl. Acad. Sci. USA 88:5949-5953 (1991)).
The second subgroup is made up of the high molecular weight, receptor-linked PTPs, termed RPTPs. RPTPs consist of (a) an intracellular catalytic region, (b) a single transmembrane segment, and (c) a putative ligand-binding extracellular domain. The structures and sizes of the putative ligand-binding extracellular "receptor" domains of RPTPs are quite divergent. In contrast, the intracellular catalytic regions of RPTPs are highly homologous. All RPTPs have two tandemly duplicated catalytic phosphatase homology domains, with the prominent exception of an RPTP termed HPTP.beta., which has only one catalytic phosphatase domain. (Tsai et al., J. Biol. Chem. 266:10534-10543 (1991)).
One example of RPTPs is a family of proteins termed leukocyte common antigens (LCA) (Ralph, S. J., EMBO J. 6:1251-1257 (1987)) which are high molecular weight glycoproteins expressed on the surface of all leukocytes and their hemopoietic progenitors (Thomas, Ann. Rev. Immunol. 7:339-369 (1989)). A remarkable degree of similarity exists in the sequences of LCA from several species (Charbonneau et al., Proc. Natl. Acad. Sci. USA 85:7182-7186 (1988)). LCA has been referred to in the literature by different names, including T200 (Trowbridge et al., Eur. J. Immunol. 6:557-562 (1962)), B220 for the B lymphocyte form (Coffman et al., Nature 289:681-683 (1981)), the mouse allotypic marker Ly-5 (Komuro et al., Immunogenetics 1:452-456 (1975)), and more recently CD45 (Cobbold et al., Leucocyte Typing III, A. J. McMichael et al., eds., pp. 788-803 (1987)).
CD45 appears to play a critical role in T cell activation (reviewed in Weiss A., Ann. Rev. Genet. 25:487-510 (1991)). For example, T-cell clones that were chemically mutagenized and selected for their failure to express CD45 had impaired responses to T-cell receptor stimuli (Weaver et al., supra). These T-cell clones were functionally defective in their responses to signals transmitted through the T cell antigen receptor, including cytolysis of appropriate targets, proliferation, and lymphokine production (Weaver et al., supra). Other studies indicate that the PTP activity of CD45 plays a role in the activation of pp56.sup.1ck, a lymphocyte-specific PTK (Mustelin et al., Proc. Natl. Acad. Sci. USA 86:6302-6306 (1989); Ostergaard et al., Proc. Natl. Acad. Sci. USA 86:8959-8963 (1989)). These authors hypothesized that the phosphatase activity of CD45 activates pp56.sup.1ck by dephosphorylation of a C-terminal tyrosine residue, which may, in turn, be related to T-cell activation.
Another example of an RPTP is the leukocyte common antigen related molecule (LAR), initially identified as a homologue of LCA (Streuli et al., J. Exp. Med. 168:1523-1530 (1988)). Although the intracellular catalytic region of the LAR molecule contains two catalytic phosphatase homology domains (domain I and domain II), mutational analyses suggested that only domain I had catalytic phosphatase activity, whereas domain II was enzymatically inactive (Streuli et al., EMBO J. 9(8):2399-2407 (1990)). Chemically induced LAR mutants having tyrosine at amino acid position 1379 changed to a phenylalanine were temperature-sensitive (Tsai et al., J. Biol. Chem. 266(16):10534-10543 (1991)).
A recently cloned mouse RPTP, designated mRPTP.mu., was found to have an extracellular domain that shared some structural motifs with LAR. (Gebbink, M. F. B. G. et al., FEBS Lett. 290:123-130 (1991). These authors also cloned a human homologue of RPTP.mu. and localized the gene on human chromosome 18.
Two Drosophila PTPs, termed DLAR and DPTP, have been predicted based on the sequences of cDNA clones (Streuli et al., Proc. Natl. Acad. Sci. USA 86:8698-8702 (1989)). cDNAs coding for another Drosophila RPTP, termed DPTP 99A, have been cloned and characterized (Hariharan et al., Proc. Natl. Acad. Sci. USA 88:11266-11270 (1991)).
Other examples of RPTPs include RPTP-.alpha., .beta., .gamma., and .zeta. (Krueger et al., EMBO J. 9:3241-3252 (1990), Sap et al. Proc. Natl. Acad. Sci. USA 87::6112-6116 (1990), Kaplan et al., Proc. Natl. Acad. Sci. USA 87:7000-7004 (1990), Jirik et al., FEBS Lett. 273:239-242 (1990), Mathews et al., Proc. Natl. Acad. Sci. USA 87:4444-4448 (1990), ohagi et al., Nucl. Acids Res. 18:7159 (1990)). Schlessinger, PCT Publication W092/01050 (23 Jan. 1992) disclosed human RPTP-.alpha., .beta. and .gamma., and described the nature of the structural homologies among the conserved domains of these three RPTPs and other members of this protein family. The murine RPTP-.alpha. has 794 amino acids, whereas the human RPTP-.alpha. has 802 amino acids. RPTP-.alpha. has an intracellular domain homologous to the catalytic domains of other tyrosine phosphatases. The 142 amino acid extracellular domain (including signal peptide of RPTP-.alpha.) has a high serine and threonine content (32%) and 8 potential N-glycosylation sites. cDNA clones have been produced that code for the RPTP-.alpha., and RPTP-.alpha. has been expressed from eukaryotic hosts. Northern analysis was used to identify the natural expression of RPTP-.alpha. in various cells and tissues. A polyclonal antibody to RPTP-.alpha., produced by immunization with a synthetic peptide of RPTP-.alpha., identifies a 130 kDa protein in cells transfected with a cDNA clone encoding a portion of RPTP-.alpha..
Another RPTP, HePTP (Jirik et al., FASEB J. 4:82082 (1990) Abstract 2253) was discovered by screening a cDNA library derived from a hepatoblastoma cell line, HepG2, with a probe encoding the two PTP domains of LCA. The HePTP gene appeared to be expressed in a variety of human and murine cell lines and tissues.
Since the initial purification, sequencing, and cloning of a PTP, additional potential PTPs have been identified at a rapid pace. The number of different PTPs that have been identified is increasing steadily, leading to speculations that this family may be as large as the PTK family (Hunter (1989) supra).
Conserved amino acid sequences designated "consensus sequences" have been identified in the catalytic domains of known PTPs (Krueger et al., EMBO J. 9:3241-3252 (1990) and Yi et al., Mol. Cell. Biol. 12:836-846 (1992), which are incorporated herein by reference). Yi et al. aligned the catalytic phosphatase domain sequences of the following PTPs: LCA, PTP1B, TCPTP, LAR, DLAR, and HPTP.alpha., HPTP.beta., and HPTP.gamma.. This alignment includes the following "consensus sequences" (Yi et al., supra, FIG. 2(A)):
1. K C X X Y W P SEQ ID NO:1!
2. H C S X G X G R X G SEQ ID NO:2!
Krueger et al., aligned the catalytic phosphatase domain sequences of PTP1B, TCPTP, LAR, LCA, HPTP.alpha., .beta., 8, .epsilon. and .zeta., and DLAR and DPTP. This alignment includes the following "consensus sequences": (Krueger et al., supra, FIG. 7):
1. K C X X Y W P SEQ ID NO:1!
2. H C S X G X G R X G SEQ ID NO:2!
It is becoming clear that dephosphorylation of tyrosine residues can by itself function as an important regulatory mechanism. Dephosphorylation of a C-terminal tyrosine residue has been shown to activate tyrosine kinase activity in the src family of tyrosine kinases (Hunter, T. Cell 49:1-4 (1987)). Tyrosine dephosphorylation has been suggested to be an obligatory step in the mitotic activation of the maturation-promoting factor (MPF) kinase (Morla et al., Cell 58:193-203 (1989)). These observations point out the need in the art for understanding the mechanisms that regulate tyrosine phosphatase activity.
It is clear that further analysis of structure-function relationships among PTPs are needed to gain important understanding of the mechanisms of signal transduction, cell cycle progression and cell growth, and neoplastic transformation. Such understanding will also provide useful agents for regulating these processes and for treating diseases associated with their dysregulation.